Oxygen can be separated from air with the use of electrically driven oxygen separation devices. Such devices employ a membrane element having an electrolyte such as yttria stabilized zirconia that is sandwiched between a cathode electrode and an anode electrode and current collectors situated at the outer surfaces of the electrodes for applying an electric potential across the electrodes and therefore, the electrolyte. When the membrane is heated to a temperature at which oxygen ion transport can occur and the electric potential is applied to the electrodes, air contacting the cathode electrode will ionize into oxygen ions and will be transported to the anode electrode. At the anode electrode, the oxygen ions will recombine into oxygen molecules that can be collected to produce an oxygen product. Such devices have particular use for industrial applications in which ultra-high purity oxygen is required.
Although there are many forms of such devices, typically the membrane element has a layered structure employing electrolyte, electrode and current collector layers in the form of a flat plate or tube. Additionally, the membrane elements are connected by means of a manifold to collect the oxygen separated by the use of the membrane elements. The assembly can be housed within an electrically heated enclosure to heat the membranes to their operational temperature. The air is supplied to the enclosure to contact the membrane elements. For example, where tubular forms of the membrane elements are used, the tubes can be connected to a manifold and the air introduced into the heated enclosure by means of a blower or the like contact the outer surface of tubes. The separated oxygen will collect within the tubes and will be discharged from the heated enclosure through an outlet conduit connected to the manifold.
As indicated above, electrically driven oxygen separation devices have particular application where the supply of ultra-high purity oxygen is required. Potential applications include use in combustion analyzers to perform elemental analysis, use a process gas in micro-electronics fabrication, and use a purge gas in laser cutting. In such applications, the oxygen requirement will vary and when the facility is closed, there will be no requirement for the oxygen. However, it is very expensive to design such a separation device with varying oxygen flow rates that meet oxygen demand for broad customer applications. In fact, in most cases, the user will require higher oxygen flow rates for short periods of time, for example, five to eight hours. Although such oxygen separation devices can be designed to supply the oxygen at varying oxygen flow rates, the oxygen separator will be very expensive and in most cases under utilized. It is far more practical to design the oxygen separation device with a fixed oxygen flow rate for example, 0.5, 1.0, 1.5 or 2.0 standard liters per minute that operates continuously, seven days a week. Such a design will increase the utilization of the oxygen separation device while maintaining fabrication costs at a more practical lower level.
The problem with designing an oxygen separation device with a fixed low output, as has been described above, is that sufficient oxygen back up volume must also be supplied so that customer can withdraw oxygen at a high flow rate (greater than generation rate) during times of peak utilization of the device. However, oxygen storage is a challenging proposition due to the fact that operating pressure of an oxygen separation device is typically very low. To provide significant oxygen back up volume, the oxygen therefore needs to be compressed for storage in a small vessel or by provision of an additional large surge tank. Both these options have inherent disadvantages. For example, the cost of an oxygen compressor can be much higher than the oxygen separation device itself and is commercially prohibitive given the alternative of supplying the oxygen from gas cylinders. Surge tanks are less expensive compared to compressors but it takes valuable space which is of prime importance in laboratories. Moreover, in order to provide sufficient back up volume in a surge tank, the oxygen must be stored at very high pressures, for example above 500 psig. However, operating tubular membrane elements at high pressure increases tube hoop stresses within such elements leading to possible failure. Also, electrode oxidation and current collector densification can become severe at such very high pressures.
As will be discussed, the present invention provides an oxygen supply method and apparatus that employs an electrically driven oxygen separation device in a more practical and efficient manner than that contemplated in the prior art outlined above.